Deposition of microplastics in estuaries: critical review of field and experimental data from the perspective of the Ocean Marginal Filter concept

Estuaries are effective natural filters retaining a significant part of the sediments and contaminants transported by rivers towards the ocean. For these complex and dynamic environments, field studies are the key to reveal the most effective retention mechanisms. This review examines published field observations of microplastics (MPs) contamination in river/estuary/sea transitional areas worldwide from the perspective of the classical Marginal Filter concept. This general concept includes three mechanisms - gravitational sedimentation, sorption, and bio-assimilation, each including a number of contaminant-specific processes. The reviewed field studies on MPs confirm, though still incompletely and sometimes inconsistently, the importance of hydrodynamics, hetero-aggregation with environmental particles, and multifaceted interactions with biota. To obtain wider experimental basis, relevant in-situ, ex-situ, and laboratory studies were reviewed on coagulation of MPs with mineral particles under changing pH and salinity, entrapment in flocs of organic material, adhesion of smaller MPs to the surface of sand grains, co-precipitation with suspended matter. Estuarine MPs transport is suggested to be size and shape selective: millimeter-sized MPs and fibers appear to be more easily exported to the ocean. Time/scale analysis of MPs-related retention/deposition processes suggests that areas of weak hydrodynamics favor the formation of accumulation zones of MPs in all estuaries, while hetero-aggregation and interactions with biota require longer time/space scales and are sensitive to seasonal and event-specific variations of environmental conditions. Water currents, turbulence, salinity, pH, organic matter content, and biomass are the key environmental factors that are highly recommended for simultaneous measurements in future studies of MP distribution in estuaries.